Our aim has been to reveal the topographic distribution of neurotransmitter receptors on nerve and muscle cells, while relating this distribution to the development and function of synapses. To this purpose, we have used alpha-bungarotoxin as a specific probe for the visualization and quantitation of nicotinic acetylcholine receptor sites. Our recent work has emphasized a model system for studying the neural control of nicotinic acetylcholine receptor distribution on skeletal muscle fibers. This system is based on the finding that embryonic and clonal nerve cells produce a factor which induces aggregation of receptors on cultured skeletal myotubes. We have now provided evidence that: 1) the factor is a large protein; 2) it activates an energy-dependent mechanism in the myotube, probably involving cytoskeletal components; 3) a similar factor, or factors, and produced by non-cholinergic as well as cholinergic nerve cell types.